1. Field of the Invention
The present invention relates in general to a method of electroplating based on coating the surface to be electroplated with filming amines such that electroplating proceeds with greatly improved speed and efficiency, especially in low current areas.
2. Description of Related Art
The metal finishing industry utilizes many types of processes, among them the electroplating of dissimilar metals, one onto another, as well as the electroplating of similar metals. In the fabrication of circuit boards, for example, copper is plated on top of copper to selectively build up the circuits.
Regardless of the type of electroplating, the first step is cleaning the substrate metal such that little or no contamination remains to interfere with the flow of ions from the anode to the substrate metal. Contamination takes the form of oxides, greases, oils, fingerprints, and the like. The contamination must be removed and the substrate metal made sufficiently active to allow the plating function to begin. This pre-plate cleaning often takes the form of immersion in multiple hot alkaline soak solutions, followed by one or more acid soak baths to activate the metal substrate and to remove any built up oxides. Oxides form extremely rapidly during rinsing cycles between the various cleaning and activating steps, and the presence of any oxides inhibits efficient plating.
Additionally, the plating cycle itself is slow and represents one of the more time consuming and difficult steps in the production cycle. Parts with odd shapes add to the problem, because low current density areas do not plate as quickly as the rest of the part. Thus excess plating in the high current density areas results from the extended plating time and/or increased amperage needed to adequately plate the low density areas. When holes are present in the article to be plated, the deposited coating on the interior of the hole is thicker at the top and bottom of the hole than at the center of the hole. The uneven coating where the plating on the exterior of the hole is thicker than the interior of the hole is known as xe2x80x9cdog boningxe2x80x9d.
The throwing power of an electroplating bath is the ratio of the current flowing at the center of the hole to the current flowing at the ends of the hole. A plating bath which has high throwing power has relatively equal flow of current at the center of the hole and the ends of the hole. A bath with high throwing power would plate the interior of the hole evenly from top to bottom. Uneven plating of the interior of the hole of circuit boards can lead to defects in the circuit board. There is thus a need for a method that cleans and activates parts and facilitates rapid, uniform electroplating.
One aspect of the invention involves a method of electroplating a substrate. The method involves obtaining a solution containing from about 0.0003% to about 30% of a film forming amine and sufficient acid to produce a pH of less than about 6.5. The method also includes coating at least a portion of the substrate with the solution, thereby forming a film on the substrate; and plating the substrate.
Advantageously, the coating involves immersing the substrate in the solution. In an embodiment, the coating involves spraying at least a portion of the substrate with the solution. Preferably, the substrate is rinsed after the coating.
In an embodiment, the solution contains a plurality of film forming amines. Advantageously, the solution includes a plurality of acids. Preferably, the acid is phosphoric acid, sulfonic acid, hydroxyacetic acid, hydrochloric acid, sulfuric acid, sulfonic acid, formic acid, malonic acid, succinic acid, aspartic acid, mallic acid, acetic acid, citric acid, nitric acid, methane sulfonic acid, gallic acid, maleic acid, or fumaric acid.
Preferably, the solution is diluted to not less than about 1% of its initial concentration. Advantageously, the substrate is dried after the coating. In an embodiment, the film forming amine inhibits oxidation of the substrate. Preferably, the acid reduces the pH of the solution to below 4.
The film forming amine may be a fatty acid amine or a long chain alkylamine. Advantageously, the film forming amine is a primary amine, a secondary amine, a tertiary amine, an alkylamine, a C-8 through C-22 amine, a basic mono cyclic tertiary amine, a stabilized abietylamine, a diaminopropane, a dehydroabietylamine, or an ethoxylate, salt, or adduct thereof.
Preferably, the solution cleans the substrate. Advantageously, a resist layer is deposited on at least a portion of the film. In an embodiment, at least a portion of the resist layer is removed. The method may also include pattern plating the substrate. The substrate may also be contacted with an acid pickle solution containing a film forming amine, where the contacting is before plating. The coating of the substrate may be repeated after plating. Preferably, the substrate is plated again after repeating the coating. The plating may be performed in a solution which contains a film forming amine.
The embodiments of the present invention relate to a method of electroplating in which the substrate to be electroplated can be prepared for electroplating in a single step. Further, both the speed and the efficiency of electroplating are increased with the embodiments of the method, especially in low current density areas such as the interior of holes. When an embodiment of the method was used to plate steel door hinges, the steel door hinges were plated in about a quarter of the time and with greater than ⅔ reduction in energy consumption compared to conventional plating. This is even more surprising, because the embodiments of the present invention do not activate but rather passivate metals.
The embodiments of the method include the use of a solution to prepare the substrate for electroplating. The solution comprises (1) a filming amine or combination of filming amines, (2) an acid or combination of acids, and (3) water. A surfactant can optionally be added to assist in cleaning and wetting of the substrates. The surfactant can also act as an antifoaming agent. The solution of the embodiments of the present invention may also be used as a cleaner or as a substitute for an acid pickle, where additional wetting agents are not required.
For purposes of this invention, a substrate is considered to be prepared for electroplating when the substrate is clean such that little or no contamination remains to interfere with the flow of ions from the plating solution to the substrate metal and when a layer of filming amine or a combination of filming amines is present on the surface of the substrate.
In addition, the solution may be used for cleaning and priming substrates, inner layer core materials, and flexible composite materials used in the manufacture of PCBs (printed circuit boards).
A filming amine is defined as a primary, secondary, tertiary or quaternary amine or derivatives thereof in which the group(s) attached to nitrogen are sufficient to cause a solution of the amine, when placed upon a substrate having a charge, to form a continuous layer when the pH of the solution is below 7. An acid is defined as a compound that, when placed in pure water with a pH of 7, causes the pH to drop below 7. A surfactant is defined as a compound which lowers the surface tension of the solution.
A wide variety of film forming amines can be used in the present invention. Preferable filming amines may be either cationic, amphoteric or nonionic. Neutral and anionic amines may also be used but do not tend to work well unless used with a strongly acid medium. Preferable filming amines may include, but are not limited to, fatty acid amines and long chain alkylamines. Amines used in embodiments of the method of the present invention can be prepared by methods known to those skilled in the art or may be purchased from any of a number of commercial sources. Preferable amines include, but are not limited to, primary, secondary and tertiary amines; diamines; alkylamines; C8 through C22 amines; basic mono cyclic tertiary amines; abietylamines; diaminopropanes; dehydroabietylamines; and ethoxylates, salts, and adducts thereof. For example, amines can be obtained from Akzo Chemicals Inc., under the tradename Armeen for primary, secondary and tertiary amines, and under the tradename Duomeen for diamines.
Fatty acid amines can be prepared by methods known to those skilled in the art by reacting fatty acids with ammonia or ammonia derivatives. The molecular structure of fatty acid amines is characterized by a central nitrogen atom as in ammonia (NH3) having one or all of its hydrogens replaced by a fatty acid group R. A fatty acid amine may also be a quaternary amine, wherein the central nitrogen atom is bonded to four fatty acid groups. Each fatty acid group, R, is a long chain, preferably C8-C22, preferably aliphatic, alkyl group having a terminal carboxylic acid function. The alkyl chain(s) of the R group may be unsaturated or have additional substituents. Because of the number of carbon atoms in the alkyl group, this group is strongly hydrophobic. However, the nitrogen atom is hydrophilic, particularly when it has four R groups or three R groups and is protonated. When dissolved or dispersed in water or non-aqueous solvents, one portion of the molecule is strongly repelled by its surrounding solvent. This repelling force tends to orient the molecules at surfaces or interfaces or cause them to form micelles when in solution.
Preferable fatty acid amines include, but are not limited to, amines having R groups derived from animal and vegetable fatty acids or oils, such as tall oil, oleic oil, caprylic oil, and coconut oil.
More preferable filming amines are commercially available from Hercules, Inc. Resins Group under the trademark Polyrad(copyright). Polyrad(copyright) filming amines are 5- and 11-mole ethylene oxide adducts of a rosin-derived primary amine to which are added 15% and 10% of the unadducted amine. The most preferable Polyrad(copyright) filming amines are Polyrad(copyright) 0515 (the 5 mole ethylene oxide adduct) and Polyrad(copyright) 1110 (the 11 mole ethylene oxide adduct). Other more preferable filming amines include organic monobasic cyclic tertiary amines commercially available from Mona Industries, Inc. under the tradename MONAZOLINE(trademark). MONAZOLINE(trademark)s are 1-hydroxyethyl-2 alkylimidazolines. The general structure is: 
where R represents alkyl groups having 7-17 carbon atoms. Most preferable MONAZOLINE(trademark) filming amines are MONAZOLINE T(trademark) and MONAZOLINE C(trademark). Other more preferable filming amines are commercially available from Akzo Chemicals Inc. under the tradename Ethoduomeen(copyright). The filming amines sold under the tradename Ethoduomeen(copyright) are ethoxylated diamines prepared by the reaction of 3, 10, or 15 moles of ethylene oxide with Duomeen(copyright) T (N-tallow-1,3-diaminopropanes). Most preferable Ethoduomeen(copyright) filming amines are Ethoduomeen(copyright) T/13 (ethoxylated (3) N-tallowalkyl-1,3-diaminopropanes), Ethoduomeen(copyright) T/20 (ethoxylated (10) N-tallowalkyl-1,3-diaminopropanes), and Ethoduomeen(copyright) T/25 (ethoxylated (15) N-tallowalkyl-1,3-diaminopropanes).
Filming amine properties particularly useful in the present invention include, but are not limited to, hydrophobicity, fast film formation, resistance to wash-off, film persistency, oxidation inhibition, cleaning, and wetting. Hydrophobicity results from the attraction between the positive charge on the filming amine and the negative charge characteristic of most substrate surfaces. Preferable filming amines form the hydrophobic film, or layer, immediately upon contact with the substrate and are resistant to water wash-off and other film degradation. The attraction between the positively charged filming amine and negatively charged substrate surface also causes adsorption and surface modification which assists in oxidation inhibition and adhesion promotion and reduction of hydrogen embrittlement in susceptible metals and alloys. Also, the amphoteric molecular structure of the filming amine provides for molecular orientation at interfaces. This orientation reduces interfacial tension which assists in substrate cleaning and wetting. To obtain these preferred properties, the solution can use one or more filming amines.
For example, preferable filming amines that create an adequate hydrophobic layer include Polyrad(copyright), MONAZOLINE(trademark), and Ethoduomeen(copyright). More preferable filming amines that create an adequate hydrophobic layer include Polyrad(copyright) 0515, MONAZOLINE T(trademark) and Ethoduomeen(copyright) T/13. Preferable filming amines include Polyrad(copyright) and Ethoduomeen(copyright), such as Polyrad(copyright) 1110 and Ethoduomeen(copyright) T/25.
To produce the solution used in the present invention, the above-described filming amine(s) are combined with a suitable acid or combination of acids. The acid(s) should be capable of reducing the solution below a pH of 7 when introduced to the filming amine. Sufficient acid is added to the filming amine to neutralize at least the majority of the amine. It is generally preferred that sufficient acid be added to neutralize the amine. In an exemplary embodiment, sufficient acid is added to provide more acid than would be required to neutralize the amine. Preferably, sufficient acid is added to reduce the pH in the solution to less than about 6.5. More preferably, sufficient acid is added to reduce the pH in the solution to less than about 4.
Preferable acids include organic and inorganic acids. More preferable acids include phosphoric, sulfonic, and hydroxyacetic acids. Most preferable acids include hydrochloric, sulfuric, methane sulfonic and sulfamic acids. Other preferable acids include formic, malonic, succinic, aspartic, mallic, acetic, citric, nitric, phosphoric, sulfonic, methane sulfonic, sulfamic, fluoboric, and hydroxyacetic acids. Gallic, maleic and fumaric acids are the most preferable acids.
The solution preferably comprises from about 0.0003% to 30.0%, by weight, of a filming amine(s) together with an acid(s) capable of dropping the pH to pH 7 or below. More preferably, the solution comprises from about 0.0008 to about 8.0%, by weight, of a filming amine(s) together with an acid(s) capable of dropping the pH below 6.5. Most preferably, the solution comprises from about 0.02% by weight to about 0.2% by weight of a filming amine together with an acid capable of reducing the pH below about 7.
When mixed, the filming amine(s) and acid(s) form cationic molecules with a strong positive charge, which readily bond to negatively charged substrates. This bonding forms a self-limiting hydrophobic layer on the substrates. The free acid above the amount required to neutralize the amine removes contamination and oxides on the substrates and assists in strengthening the hydrophobic bond. The hydrophobic bond also inhibits acidic attack of the substrates to which it is bonded, but does not inhibit acidic attack on non-charged surfaces or oxides. Thus, the combination of the filming amine(s) and acid(s), forms a hydrophobic layer which assists in the removal of contaminants from substrate materials. More specifically, the combination of the filming amine(s) and acid(s), forms a fast film forming, wash-off resistant, film persistent, hydrophobic layer which displaces contaminants from substrates. Exposing the substrate to the combination of the filming amine(s) and acid(s) primes the substrate for plating. The combination also functions as a cleaner, oxidation inhibitor, adhesion promoter, flexibility enhancer, redeposition inhibitor, and etch enhancer.
The solution further comprises a carrier, or solvent, for the filming amine(s) and acid(s). The solvent preferably comprises deionized water. Ordinary tap water can be used, but is not preferred because of the extraneous metallic ions that may be found in tap water. Distilled water is not as preferred as deionized water only because of its relatively high cost. Organic solvents, such as but not limited to, glycol ethers and alcohols may be used in addition to water. Organic solvents are particularly useful to help solubilize filming amines and amine salts with higher molecular weights. However, organic solvents are typically considered volatile organic compounds (VOCs) by government agencies.
The solution optionally further comprises a surfactant. Although not required, the surfactant increases cleaning activity, assists in chromate removal, functions as a wetting agent, and/or reduces foaming of the solution. The surfactant lowers the surface tension of the solution, preferably to less than 70 dynes/cm2, more preferably to less than 50 dynes/cm2, and most preferably to less than 30 dynes/cm2. In general, solutions having low surface tensions wet and clean the substrate more effectively than solutions having high surface tensions. Contaminants must be removed before the filming amine can bond to the substrate. If contaminants are not present on the surface of the substrate, as when a clean part is placed in an acid pickle, no added wetting agents are generally required. Preferable surfactants are commercially available from Rhone-Poulenc under the tradename Antarox. More preferable surfactants include Antarox LF 224 and Antarox BL-214.
Additionally, an etching component can be added to the solution to aid in contaminant removal and cleaning. Preferable etching components include, but are not limited to, iron salts, peroxides, persulfates, and other oxidizers. Sodium nitrobenzene sulfonate (MBS) is an exemplary etching component. Other preferable etching components include ferric chloride and cupric chloride.
Colorants or dyes can be added to the solution to, among other purposes, identify the solution. Stabilizers can be added to the solution to, among other purposes, stabilize the solution. Stabilizers may be added, for example, when the solution includes peroxysulfuric acid as an etchant. Perfumes can be added to the solution to, among other purposes, cover or remove the odor of the solution. Also, other ingredients can be added to the solution to assist in preparing the substrate for electroplating, including but not limited to, cleaning, oxidation inhibition, redeposition inhibition, hydrogen embrittlement reduction, wetting or any combination of the above.
Thus, the preferred components of the solution used to prepare substrates in the method of the present invention comprise a filming amine(s) which is mixed with an appropriate acid(s) in amounts adequate to form a hydrophobic layer on the substrate. Water is preferably used as the solvent, and a surfactant can be optionally added to aid in the cleaning, anti-foaming or wetting of the substrates. Examples of the solution are provided below: